Information about Hertz

When describing the frequency of repetitive tasks, the value can be expressed as so-many Hertz, referring to how many cycles (repetitions) occur per second.  Old text books may not actually use the name Hertz, and simply use CPS, as an abbreviation for “cycles per second”.

For example, an action that repeats itself twelve times-per-second has a frequency of twelve Hertz.

Musical tones are a heard when the ear drum vibrates within a certain frequency range.  Low pitched notes are caused by low frequency rate, and, of course, the converse is true for high pitched notes.

Named after Mr. Hertz, Hertz can also be abbreviated to HzSI units are used, when necessary, prefixing the Hertz unit name (or abbreviation), to indicate large values.  For example:

Two thousand Hertz:  2 kHz.
Three million Hertz:  3 MHz.
One thousand three hundred and twenty Hertz:  1·32 kHz, or 1k32 Hz.

The replacement of the decimal point with the SI multiplier is often used in electronics.  It helps eliminate confusion, where the decimal point can disappear on some printed material, and where different countries may use alternative characters for expressing decimal values or separating numbers above one thousand (commas and periods being used for different purposes).

Some real-world examples of Hertz values

The range of human hearing:

The normal range, for a young person (without any hearing damage) is 20 Hz to 20 kHz.  This range often decreases to around 15 kHz, or even worse, before middle age.

Because many people can't hear much beyond 15 kHz, television and radio broadcasting transmission standards were (usually) designed to only broadcast the range from 20 Hz to 15 kHz.  This is done to conserve radio broadcast spectrum (to be able to carry more stations within a given frequency space).

Various recording mediums sometimes record above 20 kHz, even though these frequencies (usually) can't be heard directly.  Doing this minimises other distortions that can occur.

UK or Australian PAL television:

Horizonal scan frequency:  15,625 Hz
Vertical scan frequency:  50 Hz
Video frame rate:  25 Hz
Video field rate:  50 Hz (see interlaced video for more information).


The standard movie camera frame rate for modern (professional) movie film is 24 fps (frames per second).  Old silent films often ran at 16 fps, and home movie cameras at 18 fps.

The projectors usually display the film at a faster rate, than the camera, to elimate flicker.  (See persistance of vision, for more information).  But it doesn't do this by running the film faster.  Instead, the projector strobes each frame, so that it's displayed more than once (usually just twice).

Traditional this was done by a rotating mechanical blade between the lamp and the film, though modern professional cinema projectors will (more often) do this by using a strobed projection lamp.


The main part of a computer, it's central processing unit (CPU), processes commands at a fixed rate.  A timing (clock) signal is generated that the CPU uses to advance through its sequence of programming.  These days, the computer clock signal is often generated in the hundreds, or thousands, of millions of cycles per second.

e.g. 400 MHz.

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